Serotonin (5-hydroxytryptamine, 5-HT) is widely distributed in animals and plants, occurring in vertebrates, fruits, nuts, and venoms. A number of congeners of serotonin are also found in nature and have been shown to possess a variety of peripheral and central nervous system activities. Serotonin may be obtained from a variety of dietary sources; however, endogenous 5-HT is synthesized in situ from tryptophan through the actions of the enzymes tryptophan hydroxylase and aromatic L-amino acid decarboxylase. Both dietary and endogenous 5-HT are rapidly metabolized and inactivated by monoamine oxidase and aldehyde dehydrogenase to the major metabolite, 5-hydroxyindoleacetic acid (5-HIAA).
Serotonin is implicated in the etiology or treatment of various disorders, particularly those of the central nervous system, including anxiety, depression, obsessive-compulsive disorder, schizophrenia, stroke, sexual dysfunction, obesity, pain, hypertension, vascular disorders, migraine, and nausea. Recently, understanding of the role of 5-HT in these and other disorders has advanced rapidly due to increasing understanding of the physiological role of various serotonin receptor subtypes. Most of the 5-HT receptors are G-protein coupled receptors, except for 5-HT3, which is an ion channel.
5-HT1A receptors are located primarily in the CNS. Agonists facilitate male sexual behavior in rats, hypotension, increase food intake, produce hypothermia, and act as anxiolytics. This receptor has also been widely implicated in depression.
The 5-HT7 receptor is the most recent addition to the burgeoning family of 5-HT receptors. 5-HT7 receptors have been cloned from rat, mouse, guinea pig, and human cDNA, and exhibit a high degree of interspecies homology (approximately 95%), but a low sequence homology with other 5-HT receptors (<40%). 5-HT7 receptors are positively coupled to adenylate cyclase when expressed in cell lines, native guinea pig hippocampus, and cultured vascular smooth muscle cells.
Preliminary data support the inference that the 5-HT7 receptor may be involved in the pathophysiology of sleep disorders, depression, (Schwartz, et al., Adv. Int. Med. 1993, 38, 81–106) and schizophrenia (Roth, et al., J. Pharmacol. Exp. Ther., 1994, 268, 1403–1410). The 5-HT7 receptor stimulation has caused relaxation of the blood vessels in monkey (Leung, et al. Br. J. Pharmocol., 1996, 117, 926–930), dog (Cushing, et al. J. Pharmocol. Exp. Ther., 1996, 277, 1560–1566) and rabbit (Martin, et al., Br. J. Pharmocol., 1995, 114, 383).
Dopamine plays a major role in addiction and psychosis. Many of the concepts that apply to dopamine apply to other neurotransmitters as well. As a chemical messenger, dopamine is similar to adrenaline. Dopamine affects brain processes that control movement, emotional response, and ability to experience pleasure and pain. Regulation of dopamine plays a crucial role in our mental and physical health. Neurons containing the neurotransmitter dopamine are clustered in the midbrain in an area called the substantia nigra. In Parkinson's disease, the dopamine-transmitting neurons in this area die. As a result, the brains of people with Parkinson's disease contain almost no dopamine. To help relieve their symptoms, these patients are given L-DOPA, a drug that can be converted in the brain to dopamine.
Certain drugs are known as dopamine agonists. These drugs bind to dopamine receptors in place of dopamine and directly stimulate those receptors. Some dopamine agonists are currently used to treat Parkinson's disease. These drugs can stimulate dopamine receptors even in someone without dopamine-secreting neurons. In contrast to dopamine agonists, dopamine antagonists are drugs that bind but don't stimulate dopamine receptors. Antagonists can prevent or reverse the actions of dopamine by keeping dopamine from attaching to receptors.
Dopamine antagonists are traditionally used to treat schizophrenia and related mental disorders. A person with schizophrenia may have an overactive dopamine system. Dopamine antagonists can help regulate this system by “turning down” dopamine activity.
Cocaine and other drugs of abuse can alter dopamine function. Such drugs may have very different actions. The specific action depends on which dopamine receptors the drugs stimulate or block, and how well they mimic dopamine. Drugs such as cocaine and amphetamine produce their effects by changing the flow of neurotransmitters. These drugs are defined as indirect acting because they depend on the activity of neurons. In contrast, some drugs bypass neurotransmitters altogether and act directly on receptors. Such drugs are direct acting.
Use of these two types of drugs can lead to very different results in treating the same disease. As mentioned earlier, people with Parkinson's disease lose neurons that contain dopamine. To compensate for this loss, the body produces more dopamine receptors on other neurons. Indirect agonists are not very effective in treating the disease since they depend on the presence of dopamine neurons. In contrast, direct agonists are more effective because they stimulate dopamine receptors even when dopamine neurons are missing.
Certain drugs increase dopamine concentrations by preventing dopamine reuptake, leaving more dopamine in the synapse. An example is the widely abused stimulant drug, cocaine. Another example is methylphenidate, used therapeutically to treat childhood hyperkinesis and symptoms of schizophrenia.
Sensitization or desensitization normally occur with drug exposure. However, addiction or mental illness can tamper with the reuptake system. This disrupts the normal levels of neurotransmitters in the brain and can lead to faulty desensitization or sensitization. If this happens in a region of the brain that serves emotion or motivation, the individual can suffer severe consequences. For example, cocaine prevents dopamine reuptake by binding to proteins that normally transport dopamine. Not only does cocaine “bully” dopamine out of the way, it also hangs on to the transport proteins much longer than dopamine does. As a result, more dopamine remains to stimulate neurons, which causes a prolonged feelings of pleasure and excitement. Amphetamine also increases dopamine levels. Again, the result is over-stimulation of these pleasure-pathway nerves in the brain.
Dopamine activity is implicated in the reinforcing effects of cocaine, amphetamine and natural rewards. However, dopamine abnormalities are also believed to underlie some of the core attention deficits seen in acute schizophrenics.